Different expressions of femininity, it's all personal preference. I have an appreciation for more masculine women, feminine women, and everything in between.
From the purely shallow perspective of looks i'd go for the girl in the colour photo. But maybe that doesn't mean much coming from a straight male. From what you have told us (barely anything) about their personalities i'd also go with the stoner over the budding marine...just as we'd probably have more in common. Don't even know why I responded as you should be able to make a better decision than predominantly heterosexual male strangers over the internet but that is just my 0.02.
Well, there's a lot of reasons. They look different. They smell different. They, generally, have a different sort of personality. They have different primary sexual characteristics. They have different secondary sexual characteristics. Masculine/feminine doesn't define male or female. Masculine girls are still girls. Feminine boys are still boys. Gender roles are stupid.